Split Spring Fuel Rod Support Grid for Fretting Mitigation
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Solution Overview
Problem
Conventional nuclear reactor fuel assembly grids face challenges in maintaining adequate fuel rod support and heat transfer efficiency due to dimensional changes and vibrations, leading to fretting and potential cladding failure, while also requiring careful design to minimize scratching and galling during rod loading.
Innovation Solution
The improved grid design features a lattice pattern with vertically elongated springs that form eight co-planar point contacts for fuel rod support, reducing the risk of fretting and scratching, and incorporates a bordering strap for structural integrity, with the springs being either integral or separate from the base strap, and attached at specific points to minimize pressure drop and enhance manufacturing simplicity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional rigid dimples and flexible springs are used to support fuel rods, then adequate rod support is provided, but grid-to-rod fretting occurs due to vibration and dimensional changes
Solution Approach 1:
The spring is divided into multiple segments or loops along its length, allowing each segment to independently deform and absorb vibrational energy. This segmentation enables the spring to maintain continuous contact with the fuel rod while accommodating dimensional changes and reducing fretting through distributed flexibility.
Solution Approach 2:
The spring design incorporates dynamic characteristics that allow it to adapt to changing conditions during reactor operation. The spring can deform elastically in response to thermal expansion, coolant pressure, and rod vibrations, maintaining optimal contact force throughout the fuel assembly lifecycle rather than relying on fixed rigid support.
2Reliability
If spring force is increased to maintain rod support during irradiation, then rod positioning is improved, but scratching and galling during rod loading increases
Solution Approach 1:
The spring's mechanical properties are optimized to provide appropriate contact force. The spring rate and initial compression are carefully selected to ensure sufficient rod positioning force while remaining below the threshold that would cause scratching or galling during rod insertion. Material selection and geometric parameters are adjusted to achieve the desired force-displacement characteristics.
Solution Approach 2:
The spring is pre-compressed to a controlled extent before rod insertion, creating a cushioning effect that absorbs impact forces during loading. This pre-compression ensures the spring is already engaged and providing gentle guidance force, preventing hard impacts that would cause scratching or galling while still maintaining adequate positioning.
3Reliability
If initial spring force is designed for maximum support, then adequate rod support is provided at start-up, but the spring force relaxes rapidly during irradiation
Solution Approach 1:
The spring is pre-compressed during assembly to store elastic potential energy that will be gradually released during reactor operation. This preliminary compression ensures the spring provides maximum support force at start-up while having sufficient stored energy to maintain adequate force throughout irradiation, compensating for the relaxation that occurs over time.
Solution Approach 2:
The spring's physical parameters are specifically designed to control the rate of force relaxation. By adjusting wire diameter, coil diameter, number of active coils, and material properties, the spring is engineered to provide high initial force while maintaining a controlled relaxation rate that ensures adequate support force throughout the entire fuel assembly operational lifetime.
4Reliability
If more springs and dimples are added per cell, then rod support is improved, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The spring is designed to perform multiple functions simultaneously: providing rod positioning force, absorbing vibrational energy, accommodating thermal expansion, and compensating for dimensional changes. This multi-functionality eliminates the need for separate components for each function, reducing overall grid complexity while maintaining or improving rod support performance.
Solution Approach 2:
The spring and dimple features are integrated into a unified support mechanism. Rather than treating springs and dimples as separate components that must be precisely positioned relative to each other, the design merges their functions into a single spring element that provides both the positioning force and the compliance needed for thermal and dimensional changes.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design provides enhanced fuel rod support with reduced fretting and scratching, improved heat transfer efficiency, and manufacturing advantages, including simpler production processes and reduced pressure drop, thereby enhancing the reliability and performance of nuclear fuel assemblies.
Implementation Method 1
The most extended portion of the ligaments is rounded in two directions to provide two, co-planar, point contacts of support for a fuel rod. The spring provides flexible support that accommodates dimensional changes during irradiation while reducing fretting.
Implementation Method 2
During irradiation, the initial spring force relaxes more or less rapidly, depending on the spring material and irradiation environment.
Implementation Method 3
The cladding diameter also changes as a result of the very high coolant pressure and operating temperatures and the pellets inside the rod also change their diameter by densification and swelling.
Data Source
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AI summary
A nuclear fuel assembly grid having a vertical, elongated, split spring on each wall of the cells that support fuel rods, wherein at least one wall of the cells that support fuel rods has a vertically elongated spring that extends into the cell, the vertically elongated spring having a vertical slit that separates a most extended portion of the spring into two vertically oriented ligaments with a most extended portion of the ligaments being rounded in two directions to provide two, coplanar, point contacts of support for a fuel rod.